The present invention relates to the field of micro-tribology, in particular to a test indenter for micro-tribological measurements of durability via microscratch and adhesion tests. The invention also relates to a method of microscratch testing. The invention may find its use for studying and testing durability, wear and scratch resistance, adhesion and delamination resistance of solid surfaces, coatings and films, as well as near-surface layers of various materials, including metals, composites, polymers, ceramics, etc.
Durability of surfaces of various materials is characterized by their wear and scratch resistance. If the surfaces are formed by coatings or films, another important characteristic of their durability is an adhesive strength with which the coating film is attached to either a substrate or an under-layer, and its delamination resistance. In combination, the aforementioned characteristics constitute a unique signature of the surface or coating.
The use of coating films, both thin and thick, in various industries is increasing constantly. Thin films are used extensively in such fields as magnetic and electronic materials. For example, a hard disk used in computer disk drives comprises either an aluminum alloy or a glass substrate, coated with a multi-layered structure of various materials, including a nickel-phosphorous layer of several micron thickness, magnetic layer(s) of a fraction of micron thickness, and then a carbon overcoat less than a dozen nanometer thick. Both scratch resistance of the top carbon layer and delamination resistance, or adhesion, of each of the layers are matters of great importance for the drive durability.
Another example of thin film application is microelectronic where thin films are applied to a silicon substrate and, with photolithography and etching, are formed into well defined fine lines used as conductive interconnections between elements of semiconductor chips. In this case the durability of the microelectronic devices depends on the delamination resistance, or adhesion, of thin films to their substrates. An example of a thick coating is paint, applied to various surfaces of automotive vehicles. The paint has to be scratch resistant, at the same time having good delamination resistance, or adhesion, to its metal or non-metal substrate. When paint includes two or three layers, for example an under-layer, color layer and transparent overcoat, the delamination resistance of each of the layers is an important characteristic of the durability. Another example is a coating on optical lenses, which may include anti-reflective and wear-resistant layers; the lenses durability is defined by both scratch resistance of the surface and delamination resistance, or adhesion strength, of each of the coated layers.
Therefore, there has been continued development in the art to evaluate surface durability by measuring such surface properties as resistance to scratch, or wear resistance, and resistance of coating films to delamination, or adhesive strength.
The most typical test, which finds wide applications for measuring the above properties, is known as a microscratch test. It is an ideal method for characterizing the surface durability, including that of films and coatings. The microscratch test can be used for all kinds of industrial coatings from thin films in semiconductor and optical industries to decorative and protective coatings for consumer goods. The microscratch test consists in that a scratching indenter, typically either steel or diamond conical tip or stylus, is pressed into the tested material under a known constant or progressively increasing applied normal load, and a relative motion is caused between the indenter and the tested surface, while evaluating the aforementioned characteristics by monitoring friction and acoustic signals.
Known in the art is a microscratch tester of CSEM, sold by Micro Photonics, Irvine, Calif., USA. The technique involves generating a controlled scratch with a conical point indenter, either a Rockwell C diamond tip or a sharp steel tip, drawn across a coated surface under either a constant or a progressively increasing load. This is schematically shown in FIG. 1, which is a side view of the test indenter 10 on the coating 12 during the test. When the coating 12 starts to fail, the corresponding critical load is detected by means of an acoustical sensor attached to an indenter holder, friction force between the indenter and the surface, penetration depth, and by optical microscopy (not shown in FIG. 1). Once known, the critical load is used to quantify the scratch resistance and adhesion properties of the film-substrate combinations.
In this test, the indenter is held perpendicular to the surface of the material being tested, and during dragging the applied load is kept normal to the test surface. A disadvantage of such point tips is that the end of the indenter is very sharp, with an extremely small radius (of about 1 to 20 xcexcm). So, when the tip is pressed into the surface of the tested coating, it develops a very high contact pressure, and even when it does not break through the coating yet, it produces significant stresses deep in the substrate. So, the test results are affected by the properties of the substrate, which makes it impossible to accurately measure the properties of thin films and coatings.
U.S. Pat. No. 5,696,327 issued in 1997 to He Huang et al. describes a microscratch test conducted with the use of a blade-type indenter, as compared with the above mentioned point microscratch test with a conical tip. In this patent, a blade-type indenter is used to facilitate calculation of the adhesion work of delamination in a two-dimensional representation, as compared to the uni-dimensional representation in the point microscratch test. The test is carried out by pressing an indenter onto a coating and moving either blade or the test sample in relation to each other, with simultaneous application of both normal load and lateral force to the indenter.
The blade-type indenter used for the above test has a symmetrical wedge-shaped cross section with front and back attack angles equal to each other. The blade is held perpendicular to the tested surface and is made from a diamond or sapphire. Accuracy of determining the adhesion work is achieved by utilizing a blade of significant width, so that the data is taken from essentially macroscopic surface areas. Such blade-type indenter is not suitable for testing small local areas or thin films or multi-layered materials. Furthermore, the method of U.S. Pat. No. 5,696,327 requires preparation of special test samples with a width narrower than the width of the blade. In addition, the test data is extremely sensitive to the blade orientation, which requires ultra-precision adjustment of the cutting edge of the blade to be parallel to the tested surface.
In the known scratch test methods, only friction and acoustic measurements were combined together, whereas another known test method with measurements of electric properties (impedance, resistance, capacitance) may be carried out separately, in combination with vertical indentation test, particularly because of non-conductivity of the diamond tips used for microscratch testing. As a result, for many material combinations the exact determination of the critical load was difficult or impossible, especially in cases of thin or multi-layered coatings.
The object of the present invention is to provide a scratch test indenter, suitable for microanalysis of coatings and thin films, which allows for simultaneous precision acoustic, electrical and mechanical measurements of the indenter-coating interactions, and thus for precision determination of the critical load of, or time till, coating failure, with improved measurement data correlation, which does not produce stresses deep in the substrate under the coating, which does not need preparation of special test samples, and is not very sensitive to the deviations in its position with respect to the tested surface. Another object is to provide a microscratch test method, which is simple, reliable, applicable for thin films and coatings due to a smaller effect of substrate properties on the test data, and which produces accurate and repeatable test results, even for multi-layered materials, by monitoring simultaneously friction, acoustic and electrical characteristics of the moving indenter-coating contact and correlating all the measured data.